In modern digital and analog cellular telecommunication systems, a cellular base station communicates with multiple mobile units while maintaining exclusive communication links with the mobile units. A base station may transmit a carrier signal that is a composite of all the exclusive carrier signals. Each receiving unit receives the composite carrier signal and extracts a desired information from the composite carrier signal. The composite carrier signal is the combined carrier signals that are transmitted on different carrier frequencies, or common carrier frequency. The composite carrier signal is amplified through a linear power amplifier that has a finite peak power capacity. The linear power amplifier should be operated in its linear region as such to prevent the composite carrier signal distortion, which also creates undesired spurious radiation. Therefore, the highest possible peak of the amplified composite carrier signal amplitude should be below the peak power capacity of the transmitter linear power amplifier.
In systems where the amplified carrier signal is a composite of multiple carrier signals, the highest possible peak amplitude, also defined as peak power, of the composite carrier signal may be substantially large. The peak power of a composite carrier signal increases as a square function of the number of carrier signals combined thereof. For example, the peak power of a composite carrier signal is approximately one hundred times larger than the average power of the individual carrier signals when a base station transmitter is transmitting a composite carrier signal comprising of ten carrier signals. In such a system, the linear power amplifier should have an arduous linear operating region, that places extreme burden on designing and keeping the linear power amplifier within its operating requirements. As a result, the cost and efficiency of the linear power amplifiers, in such a system, are substantially increased.
A clipping technique is a commonly used method for relaxing power amplifier operating requirements. In this method, if the composite carrier signal peak amplitude is above certain level, the signal would be clipped prior to being input to the power amplifier. However, a significant signal degradation often results from the clipping technique.
In another technique for controlling the peak power level of a composite carrier signal, a scheme of block coding is utilized. In the block coding scheme, each carrier signal of the composite carrier signal is coded with a code; as a result, the peak power level of the composite carrier signal may be changed to a level that the resulting amplified composite carrier signal would not be distorted. In this method, the codes are predetermined and trivially selected before any knowledge of the content of information signals; such that, it is possible to have the amplified composite carrier signal distorted at a level far above any acceptable level. The implementation of block coding scheme has been impeded due to the unpredictable results, because the codes are perfunctorily selected for each carrier signal.
Thus, a need exists for an improved method and apparatus for analyzing a composite signal to effectively reduce the peak to average amplitude ratio of a transmitted composite carrier signal.